Magnesium and its alloys are widely used in the industrial fields such as the automobile and electronic devices because they have small density and high strength. However, it is also known that magnesium has large corrosion rate due to its less-noble electrode potential. Furthermore, hydrogen evolution rate on dissolving magnesium is increased with the anodic polarization, and it’s called “Negative Difference effect (NDE)” [1-4]. For example, Baril et al. [3] proposed the following dissolution mechanism of Mg2+ for an interpretation of NDE by a local impedance measurement. Mg → Mg+ ads + e- (1) Mg+ ads ⇄ Mg2++ e- (2) Hydrogen evolution is accelerated by the following chemical reaction. Mg+ ads + H2O → Mg2+ + OH- + 1/2H2 (3) In this presentation, the dissolution and hydrogen evolution mechanisms of the pure magnesium in Na2SO4 and NaCl solutions were discussed on the basis of the following three electrochemical measurements: (1) 3D Impedance Spectroscopy, (2) Gas-Chromatographic Voltammetry [4], and (3) Channel Flow Double Electrode [5].